Besides the security aspect I can assign the port numbers.
Also, your ftp server may be able to change ports BUT could be too rough on others. I'd just go with OpenSSH since it's zero cost, etc.
I want to setup two FTP servers (each on aseperate box) behind one router with a dynamic DNS.
My problem is that there is no way to distinguish between the two servers and when I try to connect to the FTP servers I can connet only to the server I set up first, I tried to distinguish between the servers by asigning to each server a different port (and I did forworded them in the router)but it seems that FTP cant work on any other port but 21.